1. Field of the Invention
This invention relates to a method and apparatus for grinding lenses and more particularly to grinding the edge or periphery of a lens for fitting into a pair of spectacle frames.
2. Description of the Prior Art
The normal procedure for fitting a person with a pair of spectacles is for an optician to prescribe the appropriate lenses following an eye test and then for the person to choose a pair of frames into which the prescribed lenses are to be fitted. There is a wide range of sizes and shapes of frames and in order to accommodate these, the lens manufacturer produces over-size lenses to a variety of prescriptions, whether single or bi-focal, and an ophthalmic laboratory or the like will shape the required lenses to fit the chosen spectacle frames.
Lenses are shaped on a so-called lens edging machine which grinds the periphery of a lens to the required shape using a physical or electronically-memorized replica of the required shape. These lens edging machines can handle both glass lenses and lenses of synthetic plastics material and it is known first to grind a lens to the required shape using a grinding wheel, and then to finish the lens by exposing it to a V-shaped grinding wheel in order to produce an outwardly-extending, peripheral ridge or apex on the lens, which ridge is received by the associated frame, whereby each lens is located and retained in position in the pair of frames.
After a period of use, the flat grinding wheel wears and develops an inwardly-extending peripheral groove with the result that a lens is ground marginally oversize until the grinding wheel is changed, which in turn means that the V-shaped grinding wheel has to remove more material than otherwise required and is thus subjected to increased wear.